drm/ast: Enable CRTC before planes

An active cursor plane requires a valid display mode. Change the
commit_tail callback, so that it sets up the CRTC's mode before
updating planes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200914072236.19398-5-tzimmermann@suse.de